1. Field of the Invention
The present invention relates to a magnetoresistive random access memory, e.g., a spin transfer torque writing type magnetoresistive random access memory.
2. Description of the Related Art
A magnetoresistive random access memory (MRAM) uses a magnetoresistive effect element as a memory cell. The magnetoresistive effect element includes a free layer in which the magnetization direction is variable, a fixed layer in which the magnetization direction is fixed, and a nonmagnetic layer sandwiched between them. The resistance decreases when the magnetization direction in the free layer is parallel to that in the fixed layer, and increases when the former is antiparallel to the latter. The difference between these resistance states is used in information recording.
Information is read out by supplying a read current to the magnetoresistive effect element, and detecting the difference between the resistance states before and after the read current is supplied. Information is written by supplying, to the free layer of the magnetoresistive effect element, an electric current spin-polarized by the magnetic moment in the fixed layer, thereby changing the magnetization direction in the free layer. The direction of an electric current flowing through the magnetoresistive effect element controls the logic of data (U.S. Pat. No. 5,695,864). Compared to a write method using a magnetic field, this write method can have a direct effect on a nano-scale magnetic material. Therefore, no write error occurs in adjacent memory cells, and rapid magnetization reversal can be expected. There is also another advantage that the current amount necessary for data write reduces as the cell size decreases.
General requirements for the magnetoresistive random access memory are that the memory can be manufactured by a minimum number of manufacturing steps, the degree of integration can be increased by making each part as small as possible, and an arrangement and manufacturing method that hardly produce defects are applied in order to increase the yield.